The $d$ -electron configurations of ${\text{Cr}}^{2+},{\text{Mn}}^{2+},{\text{Fe}}^{2+}$ and ${\text{Co}}^{2+}$ are $d^4,d^5,d^6$ and $d^7$ respectively. Which one of the following will exhibit minimum paramagnetic behaviour?

Paramagnetic behavior is determined by the number of unpaired electrons ($n$). Since ${\text{H}}_2\text{O}$ is a weak field ligand, no pairing occurs.${\text{Cr}}^{2+}(d^4)$: $n=4$${\text{Mn}}^{2+}(d^5)$: $n=5$${\text{Fe}}^{2+}(d^6)$: $n=4$ (one pair in $t_{2g}$)${\text{Co}}^{2+}(d^7)$: $n=3$ (two pairs in $t_{2g}$)${\text{Co}}^{2+}$ has the minimum number of unpaired electrons, hence minimum paramagnetism.